US President Donald Trump today ridiculed as "phony" and "witch hunt" claims about collusion between Russia and his associates after media reports that he is being investigated for possible obstruction of justice.

"They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice," Trump tweeted, a day after The Washington Post followed by several other media outlets reported that a special counsel appointed to probe alleged Russian meddling in the 2016 presidential polls is investigating him for possible obstruction of justice.

The probe Mueller "marks a major turning point in the nearly year-old FBI investigation, which until recently focused on Russian meddling during the presidential campaign and on whether there was any coordination between the Trump campaign and the Kremlin," The Washington Post reported yesterday.

Investigators have also been looking for any evidence of possible financial crimes by Trump associates, the daily said, quoting unnamed US officials.

According to the paper, Daniel Coats, the current director of national intelligence, Mike Rogers, head of the National Security Agency, and Rogers's recently departed deputy, Richard Ledgett, agreed to be interviewed by Mueller's investigators as early as this week.

The New York Times also reported that Mueller was seeking interviews with the three officials.

Trump described such stories as witch hunt.

"You are witnessing the single greatest WITCH HUNT in American political history — led by some very bad and conflicted people!#MAGA," Trump said in another tweet.

Trump relies heavily in social media outlets, twitter in particular, to communicate directly with his followers across the country.

Trump, believes that the mainstream media has not been fair to him and outlets like The New York Times, The Washington Post and CNN is against him.

This is reflective in his several statements and tweets in the past.

At times he calls the two prominent dailies as the fake media and during the election campaign he described CNN as 'Clinton News Network'.

Mueller is overseeing a host of investigations involving people who are or were in Trump's orbit, people familiar with the probe were quoted as saying.

Significantly, Preet Bharara, the India-born former top US federal prosecutor, earlier this week had said that there was enough evidence to begin an obstruction of justice case against Trump over his alleged interference in the Russia probe.

Trump has said he is "100 per cent" willing to testify under oath on Comey meetings and tell the FBI Special Consul Robert Muller that he never asked FBI to stop investigating.